In the commercial handling of particulate materials of all kinds, it is a common practice to pass the particulate material through one or more sifting screens, for a variety of purposes, such as removal of trash and oversize particles, particle size classification, elimination of fines, etc. Typically, for this purpose, vibratory sifting screen structures are provided, in which the material to be processed is deposited upon a screen of predetermined mesh, which is controllably agitated. Undersized particles, known as "unders," pass through the screen openings and are either discharged or further processed in a subsequent screen of finer mesh. Particles too large to pass through a screen ("overs") are discharged from above the level of the screen and are collected or discarded as the case may be.
Although it is possible to achieve the necessary vibratory motion for sifting by means of positive displacement, mechanical drive mechanism, many of the more advantageous and desired forms of commercial sifters utilize vibratory drive means which are non positive displacement in character. A particularly advantageous form of such vibratory drive is found, for example, on the so-called Sweco-type sifter. In these mechanisms, the screen structure is resiliently supported by means of a plurality of circumferentially spaced, vertically oriented springs. A vibratory drive unit for this sifter mechanism is provided by means of a vertically oriented electric motor mounting upper and lower eccentric weights. By adjusting the relative angular relationship of the upper and lower eccentric weights, a variety of desirable, controlled vibratory motions is obtainable, ranging from a straight radial progression of the particulate material from the center toward the periphery of the screen, as one extreme, to various degrees of spiral progression of the material from the center toward the periphery. At the opposite extreme, typically with the eccentrics displaced 90 degrees, the particulate material will progress spirally toward the center of the screen.
Conventionally, in a commercial sifting procedure, the particulate material is controllably fed to the sifting screen by an entirely separate means, which typically may be an independently controlled and operated vibratory feeder, or possibly even manual feeding procedures. Some commercially available vibratory feeders of particulate material even utilize essentially the identical vibratory drive mechanism used in the Sweco-type sifter described above.
Notwithstanding that both sifting and feeding procedures typically involve vibratory drive mechanisms, conventional wisdom of the past has been that separate and independent apparatuses are required for feeding and sifting of particulate material. The apparent reasoning behind such conventional wisdom has been that, with non positive displacement vibratory drive means, the physical loading of the sifter screen has a consequential effect upon vibratory motion. In other words, according to conventional wisdom, a spring supported, eccentric driven sifting device, if too heavily loaded with the weight of a supply of particulate material, will have an ineffectual motion. Thus, in conventional installations, the sifting apparatus is driven independently by its own nonpositive displacement vibratory drive, and a separate apparatus is provided for delivering particulate material at reasonably controlled rates to the surface of the screen. Although the feeder itself may be subject to substantial variations in the amount of material on it at any one time, the screening mechanism tends to be isolated, at least from the effects of overloading.
In accordance with the present invention, a more complete understanding of the characteristics of particulate materials has enabled a bulk supply feeder and sifter screen to be combined for the first time in a single combination apparatus, driven by a single, common nonpositive displacement vibratory drive unit. The structure of the invention is particularly ideally suited for operation by a Sweco-type vibratory drive unit utilizing upper and lower, angularly displaced eccentric weights in conjunction with a spring supported vibratory unit.
A basic concept of the invention derives from an advantageous utilization of the principle that a particulate material, when heaped in a pile, will assume a given angle of repose (measured with respect to the horizontal). Thus, depending upon the characteristics of the particulate material, it will assume a pile of conical form. After the cone reaches a predetermined height and angle, additional material piled on top of the cone will merely slide down to the bottom. Accordingly, when particulate material is confined in a vertically elongated supply hopper, the weight "seen" by the bottom of the hopper is not necessarily the entire weight of the particulate material contained by the hopper but approximates or at least bears a relationship to the weight of a conical mound of the material whose base angle approximates the angle of repose of that material. Additional material piled into the hopper, above the imaginary conical pile, tends to slide down the sides of such imaginary conical pile, and the weight thereof is supported largely by the sidewalls of the hopper.
The apparatus of the invention takes advantage of the above described phenomenon by providing for a single vibratory unit, located directly below a supply hopper holding a bulk supply of particulate material, and providing on the vibratory unit a bottom-forming panel which is separate from but effectively constitutes the bottom of the bulk supply hopper. In the vertically elongated hopper, the bottom-forming panel is affected largely only by the weight of the imaginary conical pile of material, regardless of the height of the stored bulk material in the hopper. Accordingly, the resilient support and the eccentric drive system for the vibratory unit may be designed in the first instance to enable the desired, controlled vibratory motion to be achieved, and efficient sifting to be accomplished notwithstanding a heavy load of particulate material contained in the hopper above the vibratory unit.
In accordance with the invention, the bottom-forming panel for the supply hopper, which is actually carried by the vibratory unit mounted below and independent of the hopper, has a central discharge opening and an adjustable feed limiting device associated therewith. Accordingly, when the apparatus is in normal operation, the particulate material for the supply hopper is admitted through the discharge opening at a predetermined, limited rate, and is deposited centrally on the vibratory sifting screen. From that point, the particulate material is processed in a manner similar to conventional screening procedures.
The apparatus of the invention is unique in providing for both feeding and sifting in a single, unitary apparatus with a common, nonpositive displacement vibratory drive unit. Significant savings are realized not only in capital equipment costs, but in plant space as well. Additional, although perhaps less significant savings are realized in terms of lower operating and maintenance costs. Handling of hazardous and/or toxic materials is also simplified because of the elimination of additional handling.
For a complete understanding of the above and other features and advantages of the invention, reference should be made to the following detailed description of a preferred embodiment of the invention, and to the accompanying drawings.